Four experiments examined temporal properties of audiovisual multisensory integration. Experiment 1 measured sensitivity (d') and reaction time (RT) to 100 ms auditory (A) (1 KHz tone) and visual (V) stimuli (1 c/d Gabor), and to audiovisual (AV) combinations with stimulus onset asynchronies (SOAs) from −100 to +200 ms. Over the course of 100 trials A and V unisensory stimulus intensity was titrated to ensure a criterion sensitivity (d' = 2). For RT, significant violations of Miller's inequality (implying neural coactivation) occurred only for simultaneous AV presentation (AV SOA = 0 ms). Experiments 2–4 were identical to Experiment 1 with the following exceptions. In Experiment 2, auditory stimulus intensity was adjusted to clamp performance in auditory-only trials at a d' = 2 while visual stimulus contrast was tripled to produce a visual sensitivity of d' [[gt]] 4. Relative to Experiment 1 mean visual RT decreased by 99 ms, and AV SOAs producing neural coactivation expanded to include those for which the visual stimulus preceded the auditory stimulus by 60 ms. In Experiment 3, visual stimulus contrast was adjusted to clamp performance in visual-only trials at a d' = 2 while auditory stimulus intensity was tripled to produce an auditory sensitivity of d' [[gt]] 4. Relative to Experiment 1 mean auditory RT decreased by 106 ms, and AV SOAs producing neural coactivation expanded to include those for which the auditory stimulus preceded the visual stimulus by 60 ms. In experiment 4 the intensity of both A and V stimuli were tripled. Although mean RT to both stimuli decreased by [[gt]]100 ms relative to Experiment 1, neural coactivation occurred only for AV SOA=0 ms. Neural coactivation was, however, significantly greater than in Experiment 1. These results have implications for neural coactivation models and the generality of the inverse effectiveness rule.